This invention relates to telecommunications access systems and equipment.
Systems for carrying data at increased rates are being developed, and offer the chance to extend the range of services which customers can access using the terminal in their homes. Thus, while existing terminals can handle services according to the ISDN specification such as facsimile transmission or computer communication in addition to speech, higher data rates would offer the chance to transmit television signals into the user terminal as well as to permit interactive viewing.
A problem arises as to how to combine the higher data rate services such as ATM (Asynchronous Transfer Mode) with the existing lower data rate services such as ISDN services. In ATM, information is contained in fixed sized data packets known as cells, typically 53 byte long, which are long enough for data but short enough for delay sensitive devices (ITU Recommendations I-150).
Among the access methods being developed for high speed data transfer for terminal users are VDSL (Very High Speed Digital Asymmetrical Subscriber Line), HDSL (High Speed Digital Subscriber Line) and ADSL (Asymmetric Digital Subscriber Line). ADSL is described in ANSI Reference T1.413.
Typically, VDSL could operate with a data rate of up to 25 Mbit/sec in one direction and 3 Mbit/sec in the other direction. Typically, HDSL has data rates up to 2 Mbit/sec in each direction and ADSL has rates up to 6 Mbit/sec in one direction, and a few 100 kbit/sec in the other direction. Compared to this, basic rate ISDN is generated typically at 144 kbit/sec.
The applicants considered combining the ATM and the ISDN elements in several ways.
Referring to FIG. 1, the bandwidth occupied by analogue telephony is as indicated by xe2x80x9cATxe2x80x9d and the bandwidth for ISDN is indicated xe2x80x9cISDNxe2x80x9d. The applicants considered transmitting the VSDL signal at higher frequencies than the ISDN, i.e. not as shown in FIG. 1. The problem with this is that ISDN uses up spectrum fairly inefficiently compared to VDSL using Discrete Multi-Tone Modulation (DMT) or Carrierless Amplitude and Phase (CAP) and, if the VDSL signal were to be carried at higher frequencies, inherent restrictions as to the highest frequency which can be carried by a communication link (in particular a copper wire pair) require that the bandwidth available for VSDL would have to be restricted from the bandwidth shown in FIG. 1, reducing the capability of carrying VDSL from 25 to perhaps 12 Mbit/sec.
The applicants also considered dedicating one or more of the number of discrete carriers generated by digital multiplexed equipment (normally used to deliver one large channel) specifically for carrying the ISDN, leaving the rest to carry the ATM. However, the two elements are very separate, and an evolution later whereby both ISDN and ATM material would come from the same source, e.g. link 4, would become relatively complex.
The applicants also contemplated arranging the ISDN data in the form of the same data packets as ATM uses and then transmitting the information in this way. However, the latency of the packetization is high which would mean that the ISDN link would no longer conform to requirements.
This invention provides a method of delivering a higher data rate signal representing first services and a lower data rate signal representing second services along the same link, comprising the steps of receiving the separate signals from a communications network, and transmitting along the link the higher data rate signal in fixed size data packets within frames, the lower data rate signals being accommodated in time slots in the frames not occupied by the fixed size data packets.
The invention also provides a multiplexer for delivering a higher data rate signal representing first services and a lower data rate signal representing second services along the same link, comprising ports for reception from a communication network of the separate signals, and means for transmitting along the link the higher data rate signal in fixed size data packets within frames, the lower data rate signal being accommodated in time slots in the frames not occupied by the fixed size data packets.
The invention also provides network terminating equipment for receiving a higher data rate signal representing first services and a lower data rate signal representing second services along the same link from a multiplexer, comprising means for receiving from the link the higher data rate signals in fixed size data packets within frames, the lower data rate signal being accommodated in time slots in the frames not occupied by the fixed size data packets, and means for extracting the first services and the second services.
The invention avoids the disadvantages noted above. The lower data rate signal, e.g. ISDN, does not suffer from much delay, and it is possible for the system to be designed so that it can migrate later when the lower data rate signal, e.g. ISDN and the higher data rate signal, e.g. ATM come from the same source.
The time slots which accommodate the lower data rate signal may be part of a data packet containing a synchronization pattern, i.e. a framing data packet.
Advantageously the data packets transmitted across the link include a field containing a number which changes from packet to packet in a regularly repeating sequence, the receipt of the packet number in the sequence being acknowledged. The system may be such that, in the event that the next packet in the sequence is not received across the link, the sequence being transmitted is restarted at that next packet.